1. Field of the Invention
This invention relates to a novel method of forming an oxide barrier junction for Josephson junction devices. More particularly, the present invention relates to a method of making oxide barrier junctions for Josephson junction devices which have uniform superconducting current characteristics.
2. Description of the Prior Art
Josephson junction devices are currently being produced which comprise base electrodes and counter electrodes of superconducting materials. The superconducting material electrodes are separated by an insulation layer. The insulation layer is provided with a plurality of small windows or apertures where the electrode materials are only separated by a very thin junction barrier oxide layer having a thickness of approximately 50 angstmoms. When a current is applied that exceeds the critical current of the Josephson junction device, the oxide barrier layer will present a high impedance to the flow of current. Before this critical current level is reached, the Josephson junction device exhibits no measurable resistance. In order to incorporate such Josephson junction devices into logic circuitry, it is important that each of the devices having the same area oxide barrier junction be capable of switching or reaching the critical current level at the same value. In the present state of the art, it is required that the zero voltage supercurrent of different devices in a logic circuit be within plus or minus ten percent of each other to provide adequate operating margins so that a plurality of devices can be driven in logic sequence.
The present state of the manufacturing art of Josephson junction devices does not permit the manufacture of a large number of Josephson junction devices in a logic circuit without selecting individual discrete devices. Thus, it is extremely difficult or practically impossible to make integrated circuits which embody a large number of Josephson junction devices.
It is known that the thickness of the oxide junction barrier layer determines the magnitude of the zero voltage supercurrent. It is generally assumed that the thickness of the oxide is controlled solely by the cathode self-biasing voltage (CSBV) and the oxygen pressure that is maintained in the vacuum system during growth of the oxide barrier layer.
The most widely used method of forming the oxide junction barrier layer is to grow the oxide layer in the presence of a plasma glow discharge. The plasma region may be produced opposite the substrate being processed by employing radio frequency (RF) energy between the substrate work holder and an anode which may be positioned juxtaposed the substrate.
The oxygen in the region between the anode and the substrate work holder is ionized by the RF energy which causes an oxide layer to form and grow on the exposed area of the base electrode
IBM researchers have reported in the IBM Journal of Research and Development at Vol. 24, No. 2 (March, 1980) at pages 195 to 205 (see FIG. 4) that the zero voltage supercurrent of the oxide junction barrier depends only on the cathode self-biasing voltage and the pressure of the oxygen in the vacuum system. This IBM article reports that for a lead-indium-gold base electrode that the zero voltage superconducting current (I) is inversely proportional to the third power of the oxygen pressure (1/p.sup.3).
Researchers at the National Bureau of Standards have reported in the Journal of Vacuum Science and Technology at Vol. 15, No. 2 (March/April, 1978) at pages 392 to 395 (see FIG. 3) that the zero voltage supercurrent (I) for lead base electrodes is inversely proportional to the 4.3 power of the oxygen pressure (1/p.sup.4.3).
IBM researchers have also reported in the IEEE Transactions on Electron Devices, Vol. ED27, No. 10 (October, 1980) at pages 1998-2008 (see FIG. 8) that the zero voltage supercurrent (I) for niobium base electrodes is inversely proportional to the 7.06 power of the oxygen pressure (1/p.sup.7.06).
Heretofore, none of the methods explained in the above-mentioned articles have been able to be employed to make devices on large wafers or substrates in which the zero voltage supercurrent of the Josephson junction devices consistently remains within the required limits of plus or minus ten percent.
All of the above prior art articles are concerned with determining the exponential factor of the oxygen pressure during the ion (plasma) growth of the oxide barrier junction layer.
The present inventors have observed that the pressure of the oxygen and the cathode self-biasing voltage is indeed a factor but is insufficient in predicting the thickness of the oxide barrier junction layer. Consequently, it does not offer predictability in determining the zero voltage supercurrent. Initially, the rate of oxide growth at the barrier junction is very rapid as the ions bombard the surface of the base electrode. As the oxide layer completely covers the exposed surface of the base electrode being oxidized, the depth or thickness of the oxide layer continues more slowly as the ions must penetrate into the layer being oxidized. It was observed that the ions bombarding the surface being oxidized were causing the depth of the oxide layer to increase but the ions were also causing atoms of oxygen at the surface of the oxide layer to be dislodged. This process of building up the oxide layer was found to continue until the loss of surface atoms balanced the depth of penetration of the ions which cause the oxide growth to become stabilized. Thus, there are two processes going on simultaneously until the thickness of the oxide layer stabilizes.
It was further observed that in the case of lead-indium alloys and niobium that more than one oxide of the material being oxidized was being created and that the chemical composition of the oxide barrier layer being formed was too complex to be analyzed and explained with balanced chemical equation analysis.
Heretofore, it was not fully known what variables other than the pressure and voltage were effecting the zero voltage supercurrent.
It would be desirable to provide a method of making oxide barrier junction layers for Josephson junction devices on base electrodes which eliminates the deviations in the zero voltage supercurrent which have occurred in the prior art processes.